1. Field of the Invention
The present invention relates to an optical waveguide device and more particularly, to an optical waveguide device such as an optical modulator or an optical switch in which a Mach-Zehnder type waveguide is formed on a substrate.
2. Description of Related Art
In the fields of optical communications and optical measurements, an optical intensity modulator or an optical switch having an optical waveguide in which a Mach-Zehnder type (MZ type) structure is formed on a substrate of lithium niobate (LN), semiconductor material, or the like has been widely used.
The optical modulator having the MZ type structure includes a modulation electrode disposed along an MZ type waveguide, changes the optical phase between branched waveguides (arms) of the MZ type waveguide with a voltage applied to the modulation electrode, and controls ON/OFF states of light on the basis of an interference phenomenon in an optical coupler on the output side.
An example of an MZ type structure of the optical coupler which has the simplest structure and which can be easily manufactured is an MZ type structure in which the optical coupler has a Y-branched structure. In the waveguide having the Y-branched structure, light is guided to an output waveguide when the phases of the light input to the optical coupler are the same in the arms, and light is radiated from the Y-branched structure when the phases are opposite to each other, whereby an ON/OFF operation is performed.
Here, when the thickness of the substrate formed of LN or semiconductor material is equal to or less than 15 times the wavelength of an optical wave propagating in the optical waveguide, for example, equal to or less than 20 μm, the thickness of the substrate is approximately equal to the size of the optical waveguide and light radiated from the Y-branched structure considers the substrate itself as a slab waveguide and propagates in the vicinity of the output waveguide. Accordingly, radiation-mode light which is OFF light is mixed into the output waveguide to cause a crosstalk phenomenon, thereby causing a problem in that the ON/OFF extinction ratio characteristic of the optical waveguide device degrades.
In order to solve this problem, U.S. Pat. No. 5,627,929 discloses a technique of preventing a substrate from becoming a slab waveguide and improving the characteristics of an optical waveguide device by using an XY coupler as an optical coupler of an MZ type waveguide, as shown in FIG. 1. In FIG. 1, the MZ type structure includes an input waveguide 1, branched waveguides 2(6) and 3(7), and an output waveguide 8. Modulators 4 and 5 are disposed in a part of the branched waveguides. In U.S. Pat. No. 5,627,929, waveguides 9 and 10 deriving radiation-mode light are disposed in the optical coupler. Here, the dotted arrow represents the propagation direction of an optical wave.
In the technique disclosed in U.S. Pat. No. 5,627,929, as shown in FIG. 2, the waveguides are intersected so that the waist (A) of the optical coupler is in a single mode. In such a structure, since the discontinuity in the intersected portion is great, input light in the optical coupler excites higher-mode light and thus light is fluctuated after the coupling of the light. As a result, the extinction ratio of the output degrades or the complementarity of the ON-light output and the OFF-light output collapses. Accordingly, for example, when the OFF-light component is used for the DC bias monitoring control of an MZ type modulator but an appropriate bias point is set using the OFF light (radiation-mode light), the ON-light output side is in an inappropriate bias state and there is a problem in that the output characteristics optimal for the modulator cannot be obtained.